1. Field of the Invention
This invention pertains to marine seismic source and more particularly to an improved seismic source comprising a plurality of individual gas expansible sources.
2. Description of the Prior Art
Various seismic wave generators have been employed in the past, starting with controlled dynamite charges as a source of energy. One of the most promising of the various sources employed today use explosive gas mixtures which explode either in open-bottomed domes or into various enclosures.
One very successful gas exploder employing a closed enclosure is disclosed in U.S. Pat. No. 3,480,101 entitled "Seismic Wave Source Using Explosive Gas In An Expansible Enclosure", issued Nov. 25, 1969 in the name of Adelbert Berry, et al. The disclosure of this patent is incorporated by reference for all purposes.
A feature of the device disclosed in patent '101 and similar devices is that when the device is set off beneath the surface of the water, there is a primary energy shock imparted to the water and, after a period of time, there is an after-shock imparted to the water. The most popular explanation as to why and when this after-shock energy pulse occurs involves the effect of the hydrostatic pressure of water on the expansion enclosure of sleeve. That is, the sleeve expands the first time with gas explosion against the hydrostatic pressure of the water on the sleeve. This pressure causes the sleeve to contract or collapse; however, the gas in the chamber is sufficient, as there is pressure relief around the sleeve caused by the initial shock, to cause a second and lesser expansion to occur. Hence, there is an occurrence of a second shock.
Although the dynamics of gas expansion and external hydrostatic pressure and other related phenomena may not be completely understood or comprehended, the after-shock occurrence is undeniably present and very observable. Furthermore, it is also known that as the depth of the seismic source increases, the time between the primary shock and the after shock decreases, again probably because of the dynamic effect of the increased hydrostatic pressures at the greater depths.
A single primary source of the type described in U.S. Pat. No. 3,480,101 produces a pulse with a relatively uniform energy amplitude over a range of frequencies from about 40 Hz to 1200 Hz. it is known that the energy in the higher frequencies of this range are not particularly useful for exploration targets deeper than about 3,000 feet; therefore, it is desirable to expend proportionally more energy at lower frequencies than is available in known prior art sources.
A primary pulsing of energy from a seismic energy source of the type described above has both a positive portion and a negative portion immediately following as a result of reflection from the water source. The after-shock pulse of energy also includes both a positive and negative portion at a slightly later time, as mentioned above. The net effect of the two shocks of the type described occurring one after the other is that, in the frequency domain, one or more notches are established in the lower part of the frequency spectrum of interest. It is desirable to eliminate or greatly smooth the notching effect that results from operation of the source such as described above.
Sources which are activated on the surface of the water only have components of radiating energy outwardly and downwardly. A source that is activated somewhat below the surface of the water produces a wave in an upward direction, as well, which returns as a reflected or ghost pulse in a negative polarity with respect to the initial positive polarity. If there is no escaping energy at the surface at the time of reflection (i.e., no surface "blow-out"), then substantially the same amount of energy is reflected as initially produced. When there is "blow-out", then the amount of energy reflected is equal to one bar (i.e., one atmosphere of pressure). Having blow-out is desirable in a sense. A large shock produces a large positive portion, but a negative portion that does not exceed the predetermined value of one bar. Therefore, the result is a relatively large positive pulse and a relatively limited negative pulse, which is a desirable source shape.
However, it may be seen that a deeper source has to produce a greater shock to produce blow-out than a shallower source. This means that the positive and negative energy amplitude remain the same at a higher level for the deeper sources, resulting in notching and other undesirable characteristics at a higher energy level.
Preferred arrangement of individual gas exploders capable of operating in the manner just outlined are described and claimed in patent application Ser. No. 291,269, .Iadd.now U.S. Pat. No. 4,441,174, .Iaddend.which is hereby incorporated by reference for all purposes.
Another type of a gas expansible source, i.e., a source which produces an expanding gas at the point of origin of the source, is referred to as a pneumatic sound source or air gun. A major manufacturer of such sources is Bolt Technology Corporation of Norwalk, Conn., although there are other manufacturers of such equipment. The air guns are available covering a range of air chamber volumes from about 1,000 to 2,000 cubic inches and operating pressures from 200 to 2,000 pounds per square inch and even greater.
The Bolt air guns are marketed under the trademark PAR and typically includes two high pressure chambers, an upper control chamber and a lower discharge chamber. In the stable or rest position, the two chambers are sealed by a triggering piston and a firing piston mounted on a common shank. forming a shuttle. High pressure air (e.g., 2000 psi) supplied to the upper control chamber through an air hose from a shipboard compressor and "bleeds" into the lower firing chamber through an orifice in the shank of the shuttle. The gun is sealed because the area of the upper triggering piston is greater than that of the firing piston, and a net downward holding force exists. The gun is fired by actuating the solenoid with an electrical firing pulse. A "slug" of high pressure air is suddenly delivered to the bottom side of the triggering piston, thereby upsetting the force balance. The shuttle valve assembly opens at high velocity, reaches maximum stroke, and returns to its sealed position, all in a period of about ten milliseconds. During the few milliseconds the valve is open, most of the high pressure air in the discharge chamber is suddenly vented to the water through ports in the cylindrical wall. In larger air guns, four air jets from four individual ports merge to form a roughly spherical oscillating air bubble.
Gas expansible sources of both the gas exploder type and the air gun type are capable of being stacked in such a way that the positive portions of the primary acoustical waves accumulate while the negative portions do not. Moreover, the positive and negative portions of the after-shock waves (or so-called "bubbles", which is the common terminology for the after-shock waves from an air gun source) do not accumulate.
Air guns in the prior art have either been deployed singly or in a group at the same depth. However, it has been discovered by applicant that the vertical and horizontal spacing and variation in volumes of air guns produce desirable overall source signatures wherein the desirable characteristics described above are optimized.
Therefore, it is a feature of the present invention to provide an improved seismic marine source of the gas expansion type to .[.enchance.]. .Iadd.enhance .Iaddend.the positive portion of the primary shock energy therefrom while not enhancing the negative portions or the positive and negative portions of the after-shock energy by vertical and horizontal positioning of the individual sources.
It is another feature of the present invention to provide an improved seismic marine source of the gas expansion type to .[.enchance.]. .Iadd.enhance .Iaddend.the positive portion of the primary shock energy therefrom while not enhancing the negative portion and the after-shock energy by vertical and horizontal positioning and by volume selection of the individual sources.